What Is AIS? How Live Ship Tracking Actually Works
Every ship tracker you've ever seen — every moving dot on every marine map — runs on the same public radio system. It's called AIS, it was built to stop collisions, and understanding it explains both the magic and the gaps.
The system in one paragraph
AIS — the Automatic Identification System — is a VHF radio transponder carried by ships. Several times a minute, it broadcasts the vessel's identity (name, MMSI), position from GPS, speed, course, heading, and voyage details like destination and draught. Nearby ships receive these broadcasts to avoid running into each other; shore stations receive them to manage traffic. Trackers like VesselFlow read the same broadcasts and draw them on a map.
Who has to carry it
- Required (Class A): cargo ships of 300+ gross tons on international voyages, 500+ domestically, and all passenger ships — the SOLAS convention makes AIS mandatory. That's every container ship, tanker, bulker, ferry and cruise ship you'd want to follow.
- Voluntary (Class B): yachts, fishing boats and small commercial craft increasingly carry lower-power transponders.
- Not broadcasting: small pleasure craft without AIS, and naval vessels when they choose not to. No transponder, no dot on any app.
What “live” really means
A moving ship transmits every 2–10 seconds, but no consumer app redraws thousands of vessels that fast — and none needs to. Ships travel at 10–20 knots; in a minute, a fast container ship covers about a third of a nautical mile. VesselFlow refreshes the vessels on your screen roughly every 60 seconds and stamps each one with “updated X ago,” which is the honest way to do it: you always know whether you're looking at a fix from 40 seconds ago or 4 hours ago. Markers even tint green when the fix is under 30 minutes old.
Where the gaps come from
AIS is VHF radio, and VHF is line-of-sight. Shore receivers hear ships out to roughly 40–60 nautical miles; beyond that, coverage depends on satellites passing overhead, which is patchier. Practical consequences:
- Coasts and shipping lanes are well covered. Harbors, straits and busy routes update constantly.
- Ocean crossings go quiet. A ship mid-Pacific can be silent for hours or days, then reappear as it nears land. That's a coverage gap, not a sinking — our disappearing-ships guide covers this in depth.
- Stale dots are worse than no dots. VesselFlow ages positions out after 24 hours rather than showing a ship where it hasn't been all day.
Fun fact: AIS is deliberately unencrypted. Its first job is collision avoidance — a ship that hides its position defeats the point. That openness is why anyone with an iPhone can watch global shipping in near-real time.
See AIS in action with VesselFlow
A live map of broadcasting vessels, with per-ship freshness stamps. Free on iPhone & iPad.
FAQ
Do all ships have AIS?
All sizable cargo and passenger ships must carry it; many smaller vessels do voluntarily. Craft without a transponder appear on no tracker.
Is AIS tracking legal and public?
Yes — the broadcasts are unencrypted by design, because the system's first purpose is collision avoidance.
How often do ships broadcast their position?
Every 2–10 seconds when moving. VesselFlow refreshes your view about every 60 seconds, plenty for ships at sea speeds.